This is more in line with more modern design guidelines like those found in Material Design, which has become hugely popular the last couple of years.

Boostrap v3 had too few breakpoints for its grid, in my opinion, as the lowest one, xs, was at 768 px. A lot of traffic usually comes from screens narrower than that, so this has been frustrating for many developers.

So now they’ve added a new breakpoint, xl. This one takes the role lg used to have, and pushes the rest of the breakpoints downwards, making the range go all the way down to 576 px.